1. Field of the Invention
The present invention relates to a thermal image transfer recording medium, and more particularly to a thermal image transfer recording medium for use with a high speed bar code printer, comprising a support, a first ink layer formed thereon, and a second ink layer formed on the first ink layer, capable of easily attaining excellent image transfer onto an image-receiving sheet with the application of a small amount of thermal energy thereto, producing clear, abrasion-resistant images even on an image-receiving sheet with a low smoothness, such as regenerated paper with a smoothness as low as 50 to 150 sec in terms of Bekk's smoothness.
2. Discussion of Background
Recently a thermal image transfer recording system using a thermal head is widely used because of the advantages that it is noiseless, the apparatus for use in the system is relatively inexpensive and can be made small in size, the maintenance is easy, and printed images are stable in quality.
Thermal printing by use of such a thermal image transfer recording system is conducted by bringing a thermal image transfer recording medium into close contact with an image-receiving sheet by a thermal head including a number of heating elements, and activating the required heating elements corresponding to the required portions to be heated of the thermal image transfer recording medium, so that the heated portions of the thermofusible ink layer provided on a support are fused and transferred to the image-receiving sheet.
Representative examples of thermal image transfer recording media for use with such a thermal image transfer recording system are as follows:
(1) A thermal image transfer recording medium comprising a support and a thermofusible ink layer which comprises a coloring agent and a binder agent and is directly provided on the support. PA1 (2) A thermal image transfer recording medium comprising a support, a release layer provided on the support, and a thermofusible ink layer overlaid on the release layer. The release layer consists essentially of a wax component, and the thermofusible ink layer consists essentially of a coloring agent and a binder agent. PA1 1) a thermal image transfer recording medium comprising an ink layer comprising as the main components a coloring agent, resin particles with a melting point or softening point of 60.degree. to 140.degree. C., a wax component with a melting point or softening point of 70.degree. to 130.degree. C., and a water-soluble resin (Japanese Laid-Open Patent Application 63-45091); PA1 2) a thermal image transfer recording medium comprising an ink layer comprising as the main components (a) solid solution particles comprising a resin and a wax, (b) a coloring agent, and (c) a binder agent (Japanese Laid-Open Patent Application 63-84980); PA1 3) a thermal image transfer recording medium which comprises a first ink layer comprising as the main components styrene-based resin particles and a wax component, and a second ink layer overlaid on the first ink layer comprising a coloring agent, adhesiveness-imparting resin particles, and a wax (Japanese Laid-Open Patent Application 63-84981); PA1 4) a thermal image transfer recording medium having a ink layer comprising a coloring agent, thermoplastic resin particles, wax particles, and a water-soluble resin (Japanese Laid-Open Patent Application 63-89383); and PA1 5) a thermal image transfer recording medium which comprises a first ink layer and a second ink layer each comprising thermofusible resin particles (Japanese Laid-Open Patent Application 63-51180).
The binder agent contained in the thermofusible ink layer for the above-mentioned recording medium (1), the release layer of the recording medium (2), and the binder agent in the thermofusible ink layer for the recording medium (2), consist essentially of a wax component. The thermofusible ink layers for the recording media (1) and (2) and the release layer for the recording medium (2) generally further comprise a resin such as ethylene-vinyl acetate copolymer, polyamide, or polystyrene, in order to prevent the thermofusible ink layer from peeling off the support. The addition of the above-mentioned resin improves the shearing strength of the thermofusible ink layer or that of the combination of the thermofusible ink layer and the release layer.
When thermal printing is performed by use of the above-mentioned recording medium, particularly on a sheet of paper with a smoothness as low as 50 to 150 sec in terms of Bekk's smoothness, such as regenerated paper, for example, as shown in FIG. 3, when a thermal image transfer recording medium 1 which comprises a film support 2 and a thermofusible ink layer 3 provided thereon is brought into close contact with an image-receiving sheet 8 in such a manner that the thermofusible ink layer 3 comes into contact with the image-receiving sheet 8, and a thermal head 6 is brought into contact with the film support 2 of the thermal image transfer medium 1, so that heat is applied from the thermal head 6 to the thermofusible ink layer 3 through the film support 2, it is considered that an ink portion 7 of the thermofusible ink layer 3 is fused by the application of heat by the thermal head 6, transferred to the image-receiving sheet 8, and fixed thereto. However when the transferred ink portion 7 is cooled and solidified, even if it is tried to separate the solidified ink portion 7 from the film support 2, the ink portion 7 in fact still adheres to the film support 2, and is not transferred to the image-receiving sheet 8, so that the so-called voids are formed in the images obtained because of the insufficient image transfer performance of the thermal image transfer recording medium 1.
This is because the total of the shearing strength F1 between the solidified ink portion 7 and the non-heated portions 9, and the adhesion strength F2 between the film support 2 and the thermofusible ink layer 3 are larger than the image fixing force F3 of the ink portion 7 which is to be fixed onto the image-receiving sheet 8.
As the wax components for use in each of the thermofusible ink layer for the recording medium (1), the release layer, and the thermofusible ink layer for the recording medium (2), inexpensive petroleum waxes with low loaded needle penetrations are conventionally employed.
In the case where such petroleum waxes are employed as the wax component for the thermofusible ink layer or for the release layer, the transferred ink images are easily blurred or come off the image-receiving sheet when touched with the finger or scratched with a pen-type scanner, which would cause a serious problem when used in the field of industrial bar codes.
Furthermore, in the case where the above-mentioned thermal image transfer recording medium (1) is used for image recording on an image-receiving sheet, a satisfactory image quality cannot be obtained unless the image-receiving sheet has a smoothness as high as 1000 sec or more in terms of Bekk's smoothness.
Moreover, a large amount of a pigment such as carbon black has to be contained in the thermofusible ink layer of the above-mentioned recording medium (1) to obtain an appropriate image density, so that the pigment tends to be exposed on the image-transferred surface of the image-receiving sheets. Therefore, when the image-receiving surface comes into contact with the finger, or is scratched with a corrugated card board or a pen-type scanner, the image receiving sheet is smeared with the pigments and the bar codes printed on the image transfer sheet become illegible.
On the other hand, when the thermal image transfer recording medium (2) is employed, the image transfer to an image-receiving sheet can be easily performed because of the presence of the release layer to produce high quality images even if the image-receiving sheet has a smoothness 400 to 500 sec in terms of Bekk's smoothness.
However, these image-receiving sheets are expensive and disadvantageous from the viewpoint of running cost since such image-receiving sheets with a smoothness of 400 to 500 sec in terms of Bekk's smoothness are, for example, resin-coated plain paper and a calendered paper.
Moreover, in the release layer used in the thermal image transfer recording medium (2), a wax component with relatively large endotherm is employed. Therefore, it is difficult to apply a sufficient amount of thermal energy to the release layer for fusing and transferring the release layer to the image receiving sheet for use with a high-speed printer. Therefore, this type of recording medium has the risk that improper printing is caused in particular in high speed recording.
Furthermore, the presence of the release layer in the recording medium (2) prevents the pigment in the ink layer from being exposed on the surface of the obtained image after the printing operation. Therefore, the recording medium (2) is superior to the recording medium (1) with respect to the frictional resistance of the images obtained. However, the wax component contained as the main component in the release layer is too soft to obtain images with satisfactory frictional resistance. These cause serious problems when such recording media are used in industries.
In order to improve the frictional resistance of the image obtained by use of the conventional thermal image transfer recording media, the following media have been proposed:
In the above-mentioned conventional thermal image transfer recording media (1) to (4), each component for the ink layers is in the form of particles, so that a printed area and a non-printed area in each ink layer can be sharply separated in the course of image transfer, thus the resolution of the transferred image can be improved. However these recording media have the shortcomings that the thermofusible ink layer tends to be easily peeled off the support because of the poor adhesion thereof to the support, smearing the background of printed images with the peeled ink layer, and that the thermosensitivity of the recording medium and the frictional resistance of the obtained images are insufficient for use in practice.
The thermosensitivity of the thermal image transfer recording medium (5) is better than the thermosensitivities of the recording media of (1) to (4), but the thermal response thereof is still insufficient for use in practice, particularly for high speed printing, and the high density recording. The frictional resistance thereof is also poor.